This invention relates to a print density control for use in a thermal printer which effects printing by application of heat from a print head to a heat-sensitive sheet.
In a conventional thermal printer, the duration of the supply of power to the print head is kept constant regardless of the level of a supply voltage to the print head. Energy E (Joule) applied to the print head can be generally represented as follows where V.sub.s (volts) is the applied voltage to the print head, T.sub.s (seconds) is the duration of the supply of power (operation) to the print head and r (ohms) is the resistance of the print head. EQU E=(V.sub.s.sup.2 /r).times.T.sub.s ( 1)
This inplies that the energy E applied to the print head varies as functions of the supply voltage V.sub.s to the print head and the duration T.sub.s of voltage supply (conduction) to the print head. Should the duration T.sub.s of conduction of the print head be kept constant regardless of the supply voltage V.sub.s to the print head, the energy E to the print head is constant to provide a constant print density unless the supply voltage V.sub.s varies. However, in the case where a battery having no constant voltage property, e.g. a manganese battery widely used for a variety of electric equipments, is used as a power source of the thermal printer, the supply voltage V.sub.s to the print head varies gradually as labeled V.sub.s0, V.sub.s1 and V.sub.s2 (V.sub.s0 &gt;V.sub.s1 &gt;V.sub.s2) in FIGS. 1(a), 1(b) and 1(c) as the residual capacity of the manganese battery declines. This results in the problem of decreasing density of printouts on the heat-sensible sheet.